1. Field of the Invention
The invention relates to monolithically integrated circuits, and more particularly to an air bridge which interconnects certain conductors on an integrated circuit, while crossing over others. The invention also relates to a photolithographic process incorporating an evaporation step to form an air bridge suitable for monolithically integrated circuits.
2. Prior Art
Air bridges provide a choice to the designers of monolithic integrated circuits, who in laying out point to point circuit runs, find it necessary to continue a conductive path to another point on a chip, which cannot be reached without crossing other conductors.
A second choice is to add another dielectric layer, another metallization, and via holes to get from the original metallization to the second metallization.
The solution of another dielectrically supported metallization is not always the better choice, particularly for circuits carrying radio frequency energy. The dielectric layer increases the capacitive coupling between the overlapping conductors, and may add to losses in r.f. propagation. The air bridge, on the other hand, provides a simple technique for cross overs where they are needed, and as the name implies, it is constructed to have only an air dielectric between the overlapping conductors.
The air bridge solution requires additional process steps and should avoid the danger of contaminating the existing integrated circuit. The conventional air bridge approach is to use photolithographic processing, including a sputtering and an electroplating step. The conventional process requires a large processing time, has the potential for contaminating the surfaces with the wet chemicals used in electroplating. The electroplating is not accurate and usually requires ion milling to define the critical features. The ion milling produces stringers which if not carefully removed, may cause circuit failure.
An evaporative process is preferable in these respects, and the present invention is of this nature.